I wear a Perspiration Technology headsweats sorta-cap that ties in the back, kinda like a bandanda. It fits well under a helmet -- I hardly notice it's there, even in hot weather.
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Neil FeinMay 4 '11 at 18:05

I generally wear a "headsweats" type cap/bandana as well as sunscreen on any exposed skin. This type of product has the added benefit of absorbing some or most of the sweat coming off your dome and headed for your eyes.

And yes - as other posters have mentioned, this works well under a helmet, just like a bandanna would.

I have a couple of helmet covers that work for me. There's one in my profile pic, and a large version of that pic here. Basically thin, stretchy synthetic material that goes over the whole helmet, and includes a small visor and large cotton neck flap. Down here the hole in the ozone layer makes the sun a little more enthusistic so it's even more important. I find the helmet cover cooler than anything that goes directly on my head, and the slight extra wind resistance is not an issue when touring.

I also use a headsock in the winter, but it does get toasty in the summer.

Seems like the answers cover the range of options. I typically go with "headsweats" type options with wicking fabrics and buy em only when they are on sale. On a day where I do not expect to sweat a lot, I roll with a bandana.

A couple points that haves not been mentioned in regards to sunblock:

1) Sunblock has a tendency to run and winds up in your eyes...aside from blurry vision, it is also not a very pleasant experience. If you go with sunblock definitely go with a least a Sport version which is supposed to minimize running, but my experience is that Aveeno and Neutrogena Face Lotions are best!

I've switched to a halo skullcap, it's very similar to the headsweats option, but it has a rubber sweat channelization strip, which keeps the sweat from rolling down your head right in to your eyes. This helps a lot in the east coast's hot / humid summers.
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BenzoJun 12 '14 at 14:05